1. Field of the Invention
This invention relates to shaped oxidation catalyst structures. More particularly, this invention relates to shaped oxidation catalyst structures containing catalytic material comprised of mixed oxides of vanadium and phosphorus which are suitable for the production of maleic anhydride via the partial oxidation of nonaromatic hydrocarbons in the vapor phase with molecular oxygen or a molecular oxygen-containing gas.
2. Description of the Prior Art
The physical form of a catalyst in heterogeneous catalysis is known to be an important factor governing the activity and productivity thereof. In general, a given catalyst exhibits increased activity as the particle size of the catalyst is decreased. However, in fixed-bed reactor systems, a decrease in catalyst particle size results in an increase in pressure drop across the catalyst bed. This phenomenon results from close packing of the catalyst particles in the catalyst bed. And as the pressure drop across the catalyst bed increases, the amount of reactant gas that can be passed through the catalyst bed at a fixed inlet pressure becomes limited. Conversely, as the catalyst particle size is increased to improve pressure drop across the catalyst bed, some loss of catalyst activity results from, inter alia, the effect of lower catalyst charge density, that is, quantity of catalyst per unit of reactor volume.
In an attempt to overcome the difficulties experienced with respect to catalyst activity an productivity and pressure drop across the catalyst bed, a number of catalyst shapes have been described in the prior art.
U.S. Pat. Nos. 4,370,492 and 4,370,261 describe star or ribbed rod catalyst carrier shapes as being useful for supporting active metals for the production of vinyl acetate in the gaseous phase reaction of ethylene with acetic acid and [molecular] oxygen or [molecular] oxygen-containing gases.
U.S. Pat. Nos. 4,342,603 and 4,328,130 describes a hydrocarbon conversion catalyst having substantially the shape of a cylinder having a plurality of longitudinal channels extending radially from the circumference of the cylinder and defining protrusions therebetween. The protrusions are described as having maximum widths greater than the maximum widths of the channels.
U.S. Pat. Nos. 4,133,777 and 4,116,819 describe catalysts in the shape of elongated extrudates having alternating longitudinal grooves and protrusions on the surface. Such catalysts reportedly are useful for hydrodesulfurization of residual petroleum oils, coal liquids, shale oils, and oils from tar sands.
U.S. Pat. No. 3,966,644 describes shaped porous catalysts that are defined as concave geometric cylinders which are polylobal in shape. The catalysts reportedly are useful in the hydrotreating of hydrocarbons with increased catalyst efficiency over conventionally shaped catalysts.
In U.S. Pat. No. 3,957,627, a substantially spherical catalyst having a void center and a hole extending to the external surface is described as being useful for hydrotreating a hydrocarbon feed stock containing compounds with carbon-sulfur bonds, carbon-nitrogen bonds, and/or carbon-oxygen bonds.
U.S. Pat. No. 3,347,798 discloses hollow bead catalysts which reportedly are suitable for use in a wide variety of fluidized bed reactions, for example, in the hydrogenation, oxidation, dehydrogenation, dehydration, polymerization, condensation, amination, reduction of aromatic nitro compounds, cracking, refining and reforming of hydrocarbons, alkylation of hydrocarbons or their derivatives, and also aromatic amino, nitro and aminonitro compounds by reduction or reaction with alcohols, and also the production of alkanolamines, diamines, diphenylamine, and imines.
In spite of the foregoing, however, catalysis remains basically an inexact science, that is, an empirical art unenlightened by rules decreeing certainty and predictability. Thus, the directional effect of catalyst shape on a particular catalytic reaction with a particular catalytic material is not predictable. Primarily, this is due to each catalytic reaction having unique reaction kinetics and the catalyst utilized having unique forming characteristics.
The production of maleic anhydride via the partial oxidation of hydrocarbons in the vapor phase with molecular oxygen or a molecular oxygen-containing gas in the presence of a vanadium phosphorus oxide catalyst is one such reaction. Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It also is a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these varied needs.
Various oxidation catalysts and oxidation catalyst shapes and techniques have been used in the production of maleic anhydride, particularly the partial oxidation of nonaromatic hydrocarbons having at least four carbon atoms in a straight chain (or cyclic structure). In general, such catalysts contain mixed oxides of vanadium and phosphorus. More particularly, such catalysts wherein the valence of the vanadium is between about +3.8 and +4.8 are considered as being especially well-suited for the production of maleic anhydride from saturated hydrocarbons having at least four carbon atoms in a straight chain. In many instances, such catalysts also contain added promoter elements which are considered to exist in the catalysts as the oxide.
U.S. Pat. No. 4,283,307 describes an oxidation catalyst structure for the production of maleic anhydride which comprises a cylinder having a bore therethrough and is further described as consisting essentially of catalytic material comprised of a phosphorus, vanadium, oxygen complex.
U.S. Pat. Nos. 4,181,628 and 4,178,298 describe (solid) pellets (cylinders) as a suitable oxidation catalyst structure for the production of maleic anhydride.
U.S. Pat. No. 4,632,915 discloses catalysts comprising phosphorus, vanadium and oxygen, and a promoter component containing each of iron and lithium which are useful for the partial oxidation of nonaromatic hydrocarbons, particularly n-butane, with molecular oxygen or a molecular oxygen-containing gas in the vapor phase to produce maleic anhydride in excellent yields.
U.S. Pat. No. 4,562,268 relates to a process for the production of maleic anhydride from nonaromatic hydrocarbons in the presence of a vanadium/phosphorus mixed oxide oxidation catalyst wherein the catalyst exhibits a single pass weight/weight productivity of at least 70 grams of maleic anhydride per kilogram of catalyst per hour.
U.S. Pat. No. 4,333,853 discloses a vanadium/phosphorus mixed oxide catalyst prepared by reducing vanadium substantially in the pentavalent valence state to a tetravalent valence state in the presence of a phosphorus-containing compound and in the absence of a corrosive reducing agent in an organic liquid medium capable of reducing the vanadium to a valence state less than +5, recovering the resultant vanadium/phosphorus mixed oxide catalyst precursor, drying such precursor, and calcining the precursor to obtain the active catalyst. Such catalysts reportedly are effective in the oxidation of C.sub.4 hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, or mixtures thereof to produce maleic anhydride with selectivities ranging from 58.7% to 68.1% and yields (mol %) ranging from 51.4% to 59.5%.
U.S. Pat. No. 4,315,864 relates to a process for the production of maleic anhydride from normal C.sub.4 hydrocarbons in the presence of a vanadium/phosphorus mixed oxide catalyst. The catalyst is prepared by reducing a pentavalent vanadium-containing compound in an olefinic, oxygenated organic liquid medium to a +4 valence in the absence of a corrosive reducing agent, recovering resultant catalyst precursor, drying the catalyst precursor, and calcining the precursor to obtain the active catalyst.
U.S. Pat. No. 4,312,787 describes a catalyst which comprises an inert support and a catalytically active mixed oxide material coating of vanadium and phosphorus or of vanadium, phosphorus, and uranium on the outer surface of the support in an amount greater than 50% to about 80% by weight of the combined support and oxide material. Catalysts within the scope of the claims of the patent were reported to produce maleic anhydride from n-butane in yields ranging from 53% to 62.5%, with selectivities ranging from 57.4% to 67.9%.
In U.S. Pat. No. 4,251,390, a zinc-promoted vanadium-phosphorus-oxygen catalyst is disclosed and claimed. The catalyst is prepared by reducing pentavalent vanadium in a substantially anhydrous organic medium to a lower valent state and digesting the reduced vanadium in the presence of a zinc promoter compound. The resultant catalyst is activated by bringing the catalyst to operating temperatures for the oxidation of n-butane to maleic anhydride at a rate of 5.degree. C. to 10.degree. C. per hour in the presence of a butane-in-air mixture.
In U.S. Pat. No. 4,187,235, a process is described for preparing maleic anhydride from n-butane in the presence of a vanadium-phosphorus-oxygen high surface area catalyst, that is, 10 to 100 square meters per gram (m.sup.2 /g), as determined by the BET method. The catalyst is prepared by reducing pentavalent vanadium to a valence between +4.0 and +4.6 with a substantially anhydrous primary or secondary alcohol and contacting the reduced vanadium with phosphoric acid, followed by recovering and calcining the resultant vanadium(IV) phosphate compound.
U.S. Pat. No. 4,018,709 discloses a process for the vapor phase oxidation of normal C.sub.4 hydrocarbons using catalysts containing vanadium, phosphorus, uranium, or tungsten or a mixture of elements from zinc, chromium, uranium, tungsten, cadmium, nickel, boron, and silicon. In a preferred embodiment, the catalyst also contains an alkali metal or an alkaline earth metal, especially lithium, sodium, magnesium, or barium as active components. Typically, such catalysts are prepared in concentrated (37%) hydrochloric acid.
In U.S. Pat. No. 3,980,585, a process is disclosed for the preparation of maleic anhydride from normal C.sub.4 hydrocarbons in the presence of a catalyst containing vanadium, phosphorus, copper, oxygen, tellurium, or a mixture of tellurium and hafnium or uranium or a catalyst containing vanadium, phosphorus, copper, and at least one element selected from the group of tellurium, zirconium, nickel, cerium, tungsten, palladium, silver, manganese, chromium, zinc, molybdenum, rhenium, samarium, lanthanum, hafnium, tantalum, thorium, cobalt, uranium, and tin, optionally (and preferably) with an element from Groups IA (alkali metals) or IIA (alkaline earth metals).
U.S. Pat. No. 3,888,866 discloses a process for the oxidation of n-butane at a temperature from about 300.degree. C. to about 600.degree. C. with a vanadium/phosphorus/oxygen catalyst having a phosphorus/vanadium atom ratio of 0.5-2, promoted or modified with chromium, iron, hafnium, zirconium, lanthanum, and cerium, the promoter metal/vanadium atom ratio being between about 0.0025 and about 1. The catalysts are prepared by refluxing a reaction mixture of vanadium oxide, phosphorus, a hydrogen halide (usually hydrochloric acid), and a specified promoter metal-containing compound. The resultant catalyst precursors are recovered, dried, formed into structures--spheres, for example--and calcined to produce the active catalyst.
U.S. Pat. No. 3,864,280 discloses vanadium/phosphorus mixed oxide catalyst having an intrinsic surface area of from about 7 to about 50 m.sup.2 /g. The catalysts are prepared by precipitation of a vanadium/phosphorus/oxygen complex from an essentially organic solvent medium in the absence of gross amounts of water. The resultant crystalline precipitate is activated by heating in air, followed by a 1.5 mol % butane-in-air mixture, both at elevated temperatures.
U.S. Pat. No. 3,862,146 discloses a process for the oxidation of n-butane to maleic anhydride in the presence of a vanadium-phosphorus-oxygen catalyst complex, promoted or activated with zinc, bismuth, copper, or lithium activator. The phosphorus/vanadium and activator/vanadium atom ratios are from about 0.5-5 and from about 0.05-0.5, respectively.
U.S. Pat. No. 3,856,824 discloses a process for the production of maleic anhydride by oxidation of saturated aliphatic hydrocarbons in the presence of a catalyst comprising vanadium, phosphorus, iron, oxygen, and added modifier comprising chromium combined with at least one element selected from the group consisting of nickel, boron, silver, cadmium, and barium.
European Patent Application No. 98,039 discloses a process for the preparation of vanadium-phosphorus mixed oxide catalysts, optionally containing an added promoter element selected from the group consisting of Group IA (alkali metals), Group IIA (alkaline earth metals), titanium, chromium, tungsten, niobium, tantalum, manganese, thorium, uranium, cobalt, molybdenum, iron, zinc, hafnium, zirconium, nickel, copper, arsenic, antimony, tellurium, bismuth, tin, germanium, cadmium, and lanthanides, and mixtures thereof. The catalysts, which exhibit a phosphorus/vanadium atom ratio of from about 0.8 to about 1.3 and a promoter/vanadium atom ratio from about 0.01 to about 0.5, are prepared in an organic liquid reaction medium capable of reducing the vanadium to a valence state of approximately +4 to form a nonsolubilized catalyst precursor, contacting the nonsolubilized catalyst precursor-containing organic liquid with water to form a two-phase system having an upper organic liquid phase and a lower nonsolubilized catalyst precursor-containing aqueous phase, drying the catalyst precursor, and calcining the precursor to obtain the active catalyst.
The oxidation catalysts described in the cited references disclose several well-known solid catalyst shapes commonly employed in fixed-bed vapor phase maleic anhydride production processes, for example, spheres or spheroids, tablets, and pellets. And although the prior art catalysts and catalyst shapes generally are successful in producing the desired maleic anhydride product, the commercial utility of a catalyst system and a catalytic process is highly dependent upon the cost of the catalyst employed, the conversion of the reactants, and the yield of the desired product(s), or stated differently, the actual productivity of the catalyst system. In many instances, a reduction in the cost of a catalyst system employed in a given catalytic process on the order of a few cents per kilogram or pound, or a small percent increase in the yield of the desired product, relative to the amount of catalyst required, represents a tremendous economic advantage in a commercial operation. Accordingly, research efforts are continually being made to define new or improved catalyst systems and methods and processes of making new and old catalyst systems to reduce the cost and/or upgrade the activity, selectivity, and/or productivity of such catalyst systems in such catalytic processes. The discovery of the shaped oxidation catalyst structures of the instant invention, therefore, is believed to be a decided advance in the art.